A mobile terminal as represented by a mobile phone terminal has a characteristic that a user can call and communicate while the user moves. Therefore, when the owner who holds the mobile terminal device moves away from a base station by which the mobile terminal device can communicate at the time and thus an electric wave received from the base station becomes weak, a hand over control is performed for switching the base stations from the current one to another one that is located at the destination of the owner and from which the mobile terminal device can receive a strong electric wave.
To perform a hand over, a mobile terminal device has a function for detecting the state of an electric wave received from a peripheral base station other than a base station that is currently communicating with the mobile terminal device. A smooth hand over can be realized by constant operation of this function.
An electric power supplied from the built-in power supply of the mobile terminal device is consumed to perform the hand over control and the function for detecting the state of an electric wave received from the peripheral base station other than the base station that is currently communicating with the mobile terminal device.
Moreover, a recent mobile terminal device mounts thereon an application executing unit that executes various applications, separately from a wireless processing unit for communication. The mobile terminal device consumes a large amount of power to operate these functions.
The mobile terminal device operates as described above while consuming a limited electric power supplied from the built-in power supply of the mobile terminal device. In the time of emergency as in the event of disasters, the limited electric power of power supply is consumed rapidly and the mobile terminal device stops operating if the mobile terminal device consumes the electric power in the same manner as in the normal time; because in such case, the built-in power supply cannot be charged from a power supply and the electric power cannot be changed to a new power supply charged with sufficient power.
The mobile terminal device is a useful communication tool at the time of emergency such as disasters, for example, and notifies the base station of the position of the device itself. Hence, various attempts have been made to suppress the power consumption of the built-in power supply to extend the operating time of the mobile terminal device as much as possible and to make the mobile terminal device effectively function as a communication tool at the time of disasters.
According to one conventional technique, for example, in the case of emergency such as disasters, the base station instructs the mobile terminal device that is present near the base station to switch to a power-saving mode. On receiving the instruction, the mobile terminal device intermittently transmits and receives information to and from the base station. Thus, the mobile terminal device can suppress the power consumption of the built-in power supply. Such a conventional technique is disclosed in, for example, Japanese Laid-open Patent Publication No. 2002-158609.
In the conventional technique described above, the mobile terminal device switches its modes to a power-saving mode in response to the instruction from a base station. Hence, it is necessary to modify the configurations of both the base station and the mobile terminal device.
Moreover, though in the case of widespread disaster, the power consumption of a built-in power supply can be suppressed in response to the instruction provided from a base station; in the case of disaster inflicted only on the owner of the mobile terminal device (for example, losing a way in the mountains), there would be no instruction from the base station, and hence, the power consumption of a built-in power supply cannot be suppressed.
Moreover, though the power consumption of a built-in power supply can be suppressed compared with normal times, the mobile terminal device still consumes the electric power of the built-in power supply because of the transmission/reception of unnecessary information caused by the intermittent transmission/reception of information between the mobile terminal device and the base station.